Re: [PATCH] f2fs: introduce nid cache

[Jaegeuk Kim]

On 02/11, Chao Yu wrote:
> On 2017/2/9 9:28, Jaegeuk Kim wrote:
> > On 02/08, Chao Yu wrote:
> >> On 2017/2/7 15:24, Chao Yu wrote:
> >>> Hi Jaegeuk,
> >>>
> >>> Happy Chinese New Year! :)
> >>>
> >>> On 2017/1/24 12:35, Jaegeuk Kim wrote:
> >>>> Hi Chao,
> >>>>
> >>>> On 01/22, Chao Yu wrote:
> >>>>> In scenario of intensively node allocation, free nids will be ran out
> >>>>> soon, then it needs to stop to load free nids by traversing NAT blocks,
> >>>>> in worse case, if NAT blocks does not be cached in memory, it generates
> >>>>> IOs which slows down our foreground operations.
> >>>>>
> >>>>> In order to speed up node allocation, in this patch we introduce a new
> >>>>> option named "nid cache", when turns on this option, it will load all
> >>>>> nat entries in NAT blocks when doing mount, and organize all free nids
> >>>>> in a bitmap, for any operations related to free nid, we will query and
> >>>>> set the new prebuilded bitmap instead of reading and lookuping NAT
> >>>>> blocks, so performance of node allocation can be improved.
> >>>>>
> >>>>
> >>>> How does this affect mount time and memory consumption?
> >>>
> >>> Sorry for the delay.
> >>>
> >>> Let me figure out some numbers later.
> >>
> >> a. mount time
> >>
> >> I choose slow device (Kingston 16GB SD card) to see how this option affect mount
> >> time when there is not enough bandwidth in low level,
> >>
> >> Before the test, I change readahead window size of NAT pages from FREE_NID_PAGES
> >> * 8 to sbi->blocks_per_seg for better ra performance, so the result is:
> >>
> >> time mount -t f2fs -o nid_cache /dev/sde /mnt/f2fs/
> >>
> >> before:
> >> real	0m0.204s
> >> user	0m0.004s
> >> sys	0m0.020s
> >>
> >> after:
> >> real	0m3.792s
> > 
> > Oops, we can't accept this even only for 16GB, right? :(
> 
> Pengyang Hou help testing this patch in 64GB UFS, the result of mount time is:
> 
> Before:	110 ms
> After:	770 ms
> 
> So these test results shows that we'd better not set nid_cache option by default
> in upstream since anyway it slows down mount procedure obviously, but still
> users can decide whether use it or not depending on their requirement. e.g.:
> a. For readonly case, this option is complete no needed.
> b. For in batch node allocation/deletion case, this option is recommended.
> 
> > 
> >> user	0m0.000s
> >> sys	0m0.140s
> >>
> >> b. memory consumption
> >>
> >> For 16GB size image, there is total 34 NAT pages, so memory footprint is:
> >> 34 / 2 * 512 * 455 / 8 = 495040 bytes = 483.4 KB
> >>
> >> Increasing of memory footprint is liner with total user valid blocks in image,
> >> and at most it will eat 3900 * 8 * 455 / 8 = 1774500 bytes = 1732.9 KB
> > 
> > How about adding two bitmaps for whole NAT pages and storing the bitmaps in
> > checkpoint pack, which needs at most two blocks additionally?
> > 
> > 1. full-assigned NAT bitmap, where 1 means there is no free nids.
> > 2. empty NAT bitmap, where 1 means whole there-in nids are free.
> > 
> > With these bitmaps, build_free_nids() can scan from 0'th NAT block by:
> > 
> > 	if (full-assigned NAT)
> > 		skip;
> > 	else if (empty NAT)
> > 		add_free_nid(all);
> > 	else
> > 		read NAT page and add_free_nid();
> > 
> > The flush_nat_entries() has to change its bitmaps accordingly.
> > 
> > With this approach, I expect we can reuse nids as much as possible while
> > getting cached NAT pages more effectively.
> 
> Good idea! :)
> 
> And there is another approach which do not need to change disk layout is:
> 
> We can allocate free_nid_bitmap[NAT_BLOCKS_COUNT][455] array, each bitmap
> indicates usage of free nids in one NAT block, and we introduce another
> nat_block_bitmap[NAT_BLOCKS_COUNT] to indicate each NAT block is loaded or not,
> if it is loaded and we can do lookup in free_nid_bitmap correspondingly. So I
> expect that we will load one NAT block from disk one time at most, it will:
> - not increase mount latency
> - after loading NAT blocks from disk, we will build its bitmap inside memory to
> reduce lookup time for second time

Yup, I think both of them are doable together. Meanwhile, I've written patches
which support the bitmaps for NAT pages and started to test them. Could you
write a patch to introduce free_nid_bitmap[][] as well?

Thanks,

> 
> Thoughts? Which one is preferred?
> 
> Thanks,
> 
> > 
> > Thanks,
> > 
> >>
> >> Thanks,
> >>
> >>>
> >>>> IMO, if those do not
> >>>> raise huge concerns, we would be able to consider just replacing current free
> >>>> nid list with this bitmap.
> >>>
> >>> Yup, I agree with you.
> >>>
> >>> Thanks,
> >>>
> > 
> > .
> >